source: trunk/gcc/libjava/java/util/LinkedHashMap.java

/* LinkedHashMap.java -- a class providing hashtable data structure,
   mapping Object --> Object, with linked list traversal
   Copyright (C) 2001, 2002 Free Software Foundation, Inc.

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package java.util;

/**
 * This class provides a hashtable-backed implementation of the
 * Map interface, with predictable traversal order.
 * <p>
 *
 * It uses a hash-bucket approach; that is, hash collisions are handled
 * by linking the new node off of the pre-existing node (or list of
 * nodes).  In this manner, techniques such as linear probing (which
 * can cause primary clustering) and rehashing (which does not fit very
 * well with Java's method of precomputing hash codes) are avoided.  In
 * addition, this maintains a doubly-linked list which tracks either
 * insertion or access order.
 * <p>
 *
 * In insertion order, calling <code>put</code> adds the key to the end of
 * traversal, unless the key was already in the map; changing traversal order
 * requires removing and reinserting a key.  On the other hand, in access
 * order, all calls to <code>put</code> and <code>get</code> cause the
 * accessed key to move to the end of the traversal list.  Note that any
 * accesses to the map's contents via its collection views and iterators do
 * not affect the map's traversal order, since the collection views do not
 * call <code>put</code> or <code>get</code>.
 * <p>
 *
 * One of the nice features of tracking insertion order is that you can
 * copy a hashtable, and regardless of the implementation of the original,
 * produce the same results when iterating over the copy.  This is possible
 * without needing the overhead of <code>TreeMap</code>.
 * <p>
 *
 * When using this {@link #LinkedHashMap(int, float, boolean) constructor},
 * you can build an access-order mapping.  This can be used to implement LRU
 * caches, for example.  By overriding {@link #removeEldestEntry(Map.Entry)},
 * you can also control the removal of the oldest entry, and thereby do
 * things like keep the map at a fixed size.
 * <p>
 *
 * Under ideal circumstances (no collisions), LinkedHashMap offers O(1) 
 * performance on most operations (<code>containsValue()</code> is,
 * of course, O(n)).  In the worst case (all keys map to the same 
 * hash code -- very unlikely), most operations are O(n).  Traversal is
 * faster than in HashMap (proportional to the map size, and not the space
 * allocated for the map), but other operations may be slower because of the
 * overhead of the maintaining the traversal order list.
 * <p>
 *
 * LinkedHashMap accepts the null key and null values.  It is not
 * synchronized, so if you need multi-threaded access, consider using:<br>
 * <code>Map m = Collections.synchronizedMap(new LinkedHashMap(...));</code>
 * <p>
 *
 * The iterators are <i>fail-fast</i>, meaning that any structural
 * modification, except for <code>remove()</code> called on the iterator
 * itself, cause the iterator to throw a
 * {@link ConcurrentModificationException} rather than exhibit
 * non-deterministic behavior.
 *
 * @author Eric Blake ([email protected])
 * @see Object#hashCode()
 * @see Collection
 * @see Map
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @since 1.4
 * @status updated to 1.4